replace direct calls to unlink(2) with unlink_or_warn

This helps to notice when something's going wrong, especially on
systems which lock open files.

I used the following criteria when selecting the code for replacement:
- it was already printing a warning for the unlink failures
- it is in a function which already printing something or is
  called from such a function
- it is in a static function, returning void and the function is only
  called from a builtin main function (cmd_)
- it is in a function which handles emergency exit (signal handlers)
- it is in a function which is obvously cleaning up the lockfiles

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -189,7 +189,7 @@ static void remove_tempfile(void)
 	int i;
 	for (i = 0; i < ARRAY_SIZE(diff_temp); i++) {
 		if (diff_temp[i].name == diff_temp[i].tmp_path)
-			unlink(diff_temp[i].name);
+			unlink_or_warn(diff_temp[i].name);
 		diff_temp[i].name = NULL;
 	}
 }